Yeah, I've had voices that mumble to me too. In fact, most of my voices are mumbles. Sometimes there are even multiple voices that mumble to each other.

I try to distract myself with music or YouTube when that happens. Even watching the TV helps or listening to an audiobook. Basically, you gotta block out the mumbles with some other sound. I've found it's the only way. Sorry if that doesn't help, but it's a quick fix for me. I put on headphones whenever I start hearing mumbles.